Communication devices such as cellular phones, electronic information devices, LCD displays, solar cells, and the like require a transparent conductor having good transparency and good electrical conductivity. Further, in recent years, transparent conductors of two- and three-dimensional forms having flexibility allowing bending have been demanded in view of reduction in weight and size as well as request relating to design.
Indium Tin Oxide (ITO) is primarily used as a transparent conductive material of a transparent conductive film. However, direct patterning of the ITO to a three-dimensional form is technically difficult. Even when the ITO is patterned into a two-dimensional form and then is patterned into a three-dimensional form, an electrical conductivity lowers because the ITO is fragile and does not follow extension, contraction, bending, and the like of a transfer subject or target, so that it is technically difficult to manufacture the three-dimensional form having the transparency and the electrical conductivity. Further, the ITO contains indium that is rare earth so that problems occur in connection with resource depletion, stable supply, environmental load, and the like.
Therefore, an alternate material of the ITO has been studied, and Carbon NanoTubes (CNT), metal nano-wires, and the like have been mentioned as candidates thereof. Among them, the CNT has mechanical strength and flexibility, i.e., both the features required for the transparent conductive film having a three-dimensional form. However, when the CNTs are used, a three-dimensional network structure of a plurality of CNTs which serve as conductors and are in point-contact with each other needs to be formed in a binder fixing the CNTs for obtaining the electrical conductivity as the conductive material. For forming this network structure, for example, an application method requires many steps such as selection and adjustment of solvent and dispersant for solving and dispersing the CNTs into a solution (for example, Patent Literature 1).
Graphene is another ITO alternative material. The graphene is a two-dimensional substance having a honeycomb structure in which a carbon atom is joined to adjacent three carbon atoms. For patterning the graphene having a mechanical strength and flexibility similarly to the CNT, it is possible to employ a method of patterning the graphene while masking the graphene after film deposition. This method does not require complicated steps in contrast to the application method, and significantly differs from the CNT.
As a method of depositing a film of the graphene, there is a method in which a sheet-like Cu substrate (having a thickness of 15 μm, 25 μm) that is a catalyst is wound around a columnar reactor, Chemical Vapor Deposition (CVD) is performed at 1000° C. to deposit the substrate on the Cu substrate, adhesion to a long substrate allowing a roll-to-roll method is performed, and the Cu substrate is removed by etching (e.g., non-patent documents 1 and 2). However, this method requires many steps so that damages such as crease or breakage occur in the produced graphene film, resulting in a problem of lowering a property of the graphene. Accordingly, the inventors earnestly studied to attain the present invention of manufacturing a transfer sheet in which graphene is formed on a metal thin film layer serving as a catalyst without undergoing a transfer step causing a damage to a graphene layer.